1. Field of the Invention
This invention relates to the field of devices for the protection of power electronic devices from extreme voltage or current changes and, in particular, to snubbers which protect switching devices in unipolar brushless DC motors and also return energy stored in such snubbers to the motors protected.
2. Description of the Related Art
The use of turn on and turn off snubbers for the protection of power electronic devices is not new. Snubbers in general are devices which limit overvoltage or overcurrent transients over electronic switching devices during or after the action of switching current. Snubbers have been used to protect devices against rapid changes in voltage with respect to time (dv/dt), rapid changes in current with respect to time (di/dt) and transient voltages.
Although dissipative snubbers are very common, regenerative snubbers have been proposed to return energy stored in snubber elements back to the positive voltage supply, thereby increasing system efficiency. Regenerative snubbers may be comprised of passive or active components. Regenerative snubbers using transformers have been proposed. An actively controlled regenerative snubber configuration for use in unipolar configuration brushless direct current (DC) motors is disclosed herein. The regenerative snubber is used to maintain a constant voltage across the switching devices to prevent braking of the motor through conduction of motor back electromotive force (EMF), and to return excess energy stored in the motor phase coils to the positive voltage supply. The return of energy to the positive rail is done in a manner so as to minimize conducted electromagnetic interference at the power leads.
The most common snubber configuration is the Resistor-Capacitor-Diode (RCD) snubber. This snubber configuration appears in FIG. 1. The principle of operation of the RCD snubber follows. When switch S 2 is open, the current stored in L 4 is discharged into the snubber capacitor C 6 through diode D 8. When switch S 2 is closed again, the energy stored in C 6 discharges through the resistor R 10 back into the switch S 2. As a result, the energy stored in the coil 4 is transferred to the capacitor, C 6 and through the resistor R 10. The disadvantage of the RCD snubber is the dissipation of stored energy in the form of heat. If the switching frequency of the device to which the snubber is attached is high, the amount of energy converted to heat may be excessive.
When using the RCD snubber in the control circuit of a unipolar brushless DC motor, there arise some unique challenges particularly where low voltage, high current applications are concerned. (In a unipolar brushless DC motor the current in the windings flows in one direction through the coils from a DC source to ground.) In such a system, the snubber capacitor and resistor are shared by the phases feeding the snubber network through a series of diodes. The typical RCD snubber configuration for unipolar brushless DC motors is illustrated in FIG. 2. The snubber resistor 12 is connected to the positive voltage rail 14 so that some of the energy stored in the snubber capacitor may be returned to the positive voltage rail.
In the low side drive configuration shown in FIG. 2, each particular phase has its own diode to feed the snubber network. The diodes are present to prevent the shorting out of the motor phase coils. (The low side drive configuration is called by this name since the switch S 15 is connected to the low side or bottom side of the load. The low side drive configuration is also called a boost converter since VA 13 is always greater than VS 17 when the switch S 15 is turned on and off rapidly.) Such a snubber has been proposed by Elliot et al., U.S. Pat. No. 4,678,973. The performance of the snubber resistor 12 is not optimized for all motor speeds. Therefore, at high motor speeds when there is no chopping, the snubber resistor acts as a brake. Consequently, the efficiency of the motor is adversely affected at high motor speeds. For a 600 watt (W) application,there would be an increase in losses and, therefore, the, snubber circuit would dissipate more power.
An RCD snubber network is only required for low side drive, or boost converter type motor drives. In a high side drive, or buck converter motor drive, the RCD snubber may be replaced by diodes connected to the positive rail. This configuration is illustrated in FIG. 3. This configuration is so named since the switch S119 is connected to the high side or top side of the load. It is also called a xe2x80x9cbuckxe2x80x9d converter or a xe2x80x9cdownxe2x80x9d converter as VA 21 is never greater than VS 23. In some automotive systems, however, there is a requirement for reverse voltage protection.
In the high side drive configuration illustrated in FIG. 3, if the switching devices used are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which are best suited for high power, low voltage operation (low on resistance), the reverse protection devices must be included due to the source to drain diodes inherent to the MOSFETs. These diodes will become forward biased during a reverse polarity condition. The additional reverse protection devices have a voltage drop associated with them, causing a loss in motor efficiency.
The low side drive configuration, on the other hand, requires reverse voltage protection devices. However, in this configuration, only two voltage drops are dealt with, as opposed to three in the case of the high side drive, or buck converter, configuration.
In the boost, or low side drive configuration, passive means of energy recovery through the use of catch windings has been proposed by Finney et al., The RCD Snubber Revisited. IEEE Industrial Applications Society Conference Proceedings, Toronto, Canada, 1993, pp. 1267-1273. The disadvantage of this method is the size of the transformers needed to return the energy stored in the snubber capacitor back to the positive voltage supply. The transformer may be another phase coil, or part of the same winding as the phase coil. There is a cost penalty associated with the manufacture of bifilar windings (two windings per phase), however, in addition to the difficulty in winding them.
Active snubber configurations have been proposed by Zach et al., New Lossless Turn-On and Turn-Off (Snubber) Networks for Inventors, Including Circuits for Blocking Voltage Limitation, IEEE Transactions on Power Electronics, April 1986, pp. 65-75, and by Elasser and Torrey, Soft Switching Active Snubbers for DC/DC Converters, IEEE Applied Power Electronics Conference, Dallas, Texas, 1995, pp.483-489. These snubber configurations use a transistor and an inductor to replace the snubber resistor. The purpose of the transistor and inductor is to discharge the snubber capacitor to the positive voltage rail, recycling the capacitor energy and thereby increasing system efficiency.
In unipolar brushless DC motors, the active snubber configuration may also be used to increase motor efficiency at all speeds. The added challenges with motors of this type involve dealing with motor back electromotive force (EMF), and the return of the capacitor energy to the positive rail in such a manner as to minimize conducted radio frequency noise emissions.
The circuit for the active snubber configuration is shown in FIG. 4. Inductor L is connected to the positive voltage supply 20, and the top of switch S122 is used to switch the windings at a frequency fc. Coil L may be modeled as a resistor, R124, in series with an inductor L118. As a particular phase is switched when the value of the back EMF in the coil is at a maximum, the back EMF many be modeled as a DC voltage source during this time. Back EMF voltage E 26 is in opposition to the battery voltage +V 20. For any given motor speed, the motor back EMF E, is given by:
E=kexcfx89
Where:
ke=the motor electrical constant (V radsxe2x88x921sxe2x88x921)
xcfx89=the motor speed (rads sxe2x88x921)
The bottom of L118 is connected to the snubber circuit comprised of L228, S230, and C 32, through diode D134. When L118 is turned on, a current i1 (t) flows through the phase coil, L1, though switch S122, and to ground. There will be a buildup of current in the phase coil L1. When the phase coil L1 is switched off, there will be a voltage appearing at the bottom of the coil as illustrated in FIG. 5. The magnitude of this voltage is equal to:       V    S1    =            +      V        +          L      ⁢              xe2x80x83            ⁢                        ⅆ                      i            1                                    ⅆ          t                    ⁢              xe2x80x83            ⁢              (        V        )            ⁢              xe2x80x83            ⁢              (                  by          ⁢                      xe2x80x83                    ⁢                      Lenz            '                    ⁢          s          ⁢                      xe2x80x83                    ⁢          law                )            
If the voltage across S122 is great enough, S122 will avalanche (break down and conduct the short circuit current of the device), and could be permanently damaged. Consequently, it is desired to limit the voltage across S1 to a safe level as specified in the device literature. It is also desired to minimize the switching losses in the device. As switching losses are the product of the voltage and current during turn off and turn on, the magnitude of losses will be proportional to the voltage across S1, during turn on and turn off. To maximize motor efficiency, the motor back EMF should not be conducted through the snubber network, for the snubber will act as a brake. As this is a common snubber to all phase coils, the snubber capacitor voltage must be maintained slightly higher in magnitude above the back EMF voltage for a particular motor speed.
When S122 is turned off, the phase coil begins to discharge through the snubber diode for that particular phase. Once the capacitor voltage reaches a certain value, switch S230 is turned on, and the capacitor is discharged through inductor L228. The purpose of L228 is twofold. L2 is included to slow the discharge of capacitor C 32, and to ensure that C 32 is discharged in a sinusoidal fashion for harmonic minimization of the discharge current This is done to minimize high frequency conducted and radiated noise emissions. Once the voltage across C 32 falls to below the preset level dependent on the motor speed, S230 is once again shut off.
The motor back EMF is linear with respect to motor speed. Therefore, the desired capacitor voltage will also be linear with respect to motor speed, being slightly greater than the magnitude of the motor back EMF. This is illustrated in FIG. 6. The motor for which the invention is used is a unipolar motor with the back EMF superimposed on top of the supply voltage. Consequently, the minimum desired capacitor voltage will be the supply voltage. Therefore, the desired capacitor voltage also depends on the applied terminal voltage in addition to the motor speed.
If motor input power is very high and the energy dumped into a common snubber would cause excessive temperature rise and reliability problems in the active snubber components, a separate snubber could be used for each motor phase. The general arrangement in such a case is shown in FIG. 4a. 
In general, the invention comprises an actively controlled regenerative snubber configuration for use in a unipolar brushless direct current motor comprising a snubber circuit, the snubber circuit comprising a first inductor, a first switch, and a capacitor, the first inductor, the first switch and the capacitor being connected in series to a positive voltage supply of the motor.
In an alternative embodiment, the invention comprises an actively controlled regenerative snubber configuration for use in a unipolar brushless direct current motor comprising at least one snubber circuit, each of the at least one snubber circuit comprising a first inductor, a first switch, and a capacitor, the first inductor, the first switch and the capacitor being connected in series to a positive voltage supply of the motor.
The invention also comprises a method for protecting a unipolar brushless direct current motor and returning energy to the motor by employing an actively controlled regenerative snubber configuration comprising a snubber circuit, the snubber circuit comprising a first inductor, a first switch, and a capacitor, the first inductor, the first switch, and the capacitor being connected in series to a positive voltage supply of the motor, the snubber configuration further comprising a set of a second inductor and a second switch for each phase of at least one phase of current powering the motor, each set of the second inductor and the second switch being connected to the positive voltage supply in parallel with the first inductor, the first switch, and the capacitor, the method comprising the steps of: 1) closing the second switch corresponding to a phase of the at least one phase of current, thereby allowing the phase to flow through the second inductor corresponding to the phase and increase within the second inductor; 2) opening the second switch when voltage across the second switch has reached a first predetermined value, thereby allowing the second inductor to discharge into the capacitor; 3) closing the first switch once voltage across the capacitor has reached a second predetermined value, thereby allowing the capacitor to discharge through the first inductor; 4) opening the first switch once voltage across the capacitor falls to a value below a third predetermined value; and 5) returning to step 1) to perform it again.
The invention also comprises a method for protecting a unipolar brushless direct current motor and returning energy to the motor by employing an actively controlled regenerative snubber configuration comprising at least one snubber circuit, each snubber circuit of the at least one snubber circuit comprising a first inductor, a first switch, and a capacitor, each snubber circuit corresponding to one phase of at least one phase of current powering the motor, each snubber circuit being connected in series to a positive voltage supply of the motor, the snubber configuration further comprising a set of a second inductor and a second switch for each phase of the at least one phase of current powering the motor, each set of the second inductor and the second switch being connected to the positive voltage supply in parallel with each snubber circuit, the method comprising die steps of: 1) closing the second switch corresponding to a phase of the at least one phase of current, thereby allowing the phase to flow through the second inductor corresponding to the phase and increase within the second inductor; 2) opening the second switch corresponding to the phase when voltage across the second switch corresponding to the phase has reached a first predetermined value, thereby allowing the second inductor corresponding to the phase to discharge into the capacitor corresponding to the phase; 3) closing the first switch corresponding to the phase once voltage across the capacitor corresponding to the phase has reached a second predetermined value, thereby allowing the capacitor corresponding to the phase to discharge through the first inductor corresponding to said phase; 4) opening the first switch corresponding to the phase once voltage across the capacitor corresponding to the phase falls to a value below a third predetermined value; and 5) returning to step 1) to perform it again.